What is the meaning of 'sad beauty'?

There's no slang meaning. "Sad beauty" means exactly what it says: beauty that's sad.

Beauty is often short-lived. When it is, bearing witness to it, like bearing witness to someone's beauty and mortality simultaneously, is often sad for the witness, so seeing a picture of the now-late Princess Diana holding a terminally ill child with AIDS would be an example of "sad beauty." Another example is films that are both beautiful and sad, like Sophie's Choice, Brian's Song, The Fault in Our Stars, and The Goldfinch are very beautiful and very sad films, thus exemplifying "sad beauty."

We can also imbue sadness onto beauty after we know it has met some tragic end, like when we look at any picture of Princess Diana now, not just ones with her holding terminally ill children. When my grandmother was still in her twenties, she died quite tragically. My grandmother was also very beautiful. So now when my mother and great grandmother look at pictures of her, they see her as a "sad beauty." She didn't look sad or make people sad when she was alive, but now, because of her untimely death and an overwhelming sense of loss and sadness that flowed from that, her beauty, like when seen in photographs, is imbued with sadness, evokes tremendous feelings of sadness, so this is another example of "sad beauty."

"Sad beauty" can also be beauty that does itself look innately sad or that for some aesthetic reason is heartbreaking to look at, like I would say that Irish actor Anthony Boyle (pictured below) exemplifies "sad beauty" because he's beautiful and just naturally from his big, doey eyes that pronouncedly angle outwardly downward like eyes do when someone's weeping or grief-stricken and also from how he generally holds his face, especially his mouth and eyebrows, has a look about him that's a bit heartbreaking, that's sad.

Anthony Boyle